Lateral DMOS device with dummy gate

ABSTRACT

An LDMOS transistor with a dummy gate comprises an extended drift region over a substrate, a drain region in the extended drift region, a channel region in the extended drift region, a source region in the channel region, a first dielectric layer with a first thickness formed over the extended drift region, a second dielectric layer with a second thickness formed over the extended drift region and the channel region, wherein the first thickness is greater than the second thickness, and wherein the first dielectric layer and the second dielectric layer form two steps, a first gate formed over the first dielectric layer and a second gate formed above the second dielectric layer.

This is a continuation application of U.S. application Ser. No.13/351,295, entitled “Lateral DMOS Device with Dummy Gate” which wasfiled on Jan. 17, 2012 and is incorporated herein by reference.

BACKGROUND

The semiconductor industry has experienced rapid growth due toimprovements in the integration density of a variety of electroniccomponents (e.g., transistors, diodes, resistors, capacitors, etc.). Forthe most part, this improvement in integration density has come fromshrinking the semiconductor process node (e.g., shrink the process nodetowards the sub-20 nm node). As semiconductor devices are scaled down,new techniques are needed to maintain the electronic components'performance from one generation to the next. For example, lowgate-to-drain capacitance and high breakdown voltage of transistors aredesirable for high power applications.

As semiconductor technologies evolve, metal oxide semiconductor fieldeffect transistors (MOSFET) have been widely used in today's integratedcircuits. MOSFETs are voltage controlled device. When a control voltageis applied to the gate a MOSFET and the control voltage is greater thanthe threshold of the MOSFET, a conductive channel is built between thedrain and the source of the MOSFET. As a result, a current flows betweenthe drain and the source of the MOSFET. On the other hand, when thecontrol voltage is less than the threshold of the MOSFET, the MOSFET isturned off accordingly.

MOSFETs may include two major categories. One is n-channel MOSFETs; theother is p-channel MOSFETs. According to the structure difference,MOSFETs can be further divided into three sub-categories, planarMOSFETs, lateral double diffused MOS (LDMOS) FETs and vertical doublediffused MOSFETs. In comparison with other MOSFETs, the LDMOS is capableof delivering more current per unit area because its asymmetricstructure provides a short channel between the drain and the source ofthe LDMOS.

In order to increase the breakdown voltage of the LDMOS, the gatepoly-silicon of the LDMOS may be extended to make an overlap with thedrift region of the LDMOS. Such an overlap functions as a field plate tomaintain the breakdown voltage of the LDMOS.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a simplified cross-sectional view of a lateral DMOStransistor with a dummy gate in accordance with an embodiment;

FIG. 2 illustrates a simplified top view of a lateral DMOS transistorwith a dummy gate in accordance with an embodiment; and

FIG. 3 illustrates two curves illustrating the performance differencebetween a traditional LDMOS transistor without a dummy gate and a LDMOStransistor with a dummy gate in accordance with an embodiment.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the variousembodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the present embodiments are discussed in detailbelow. It should be appreciated, however, that the present disclosureprovides many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative of specific ways to make and use the embodimentsof the disclosure, and do not limit the scope of the disclosure.

The present disclosure will be described with respect to embodiments ina specific context, a lateral double diffused metal oxide semiconductor(LDMOS) device with a dummy gate. The embodiments of the disclosure mayalso be applied, however, to a variety of metal oxide semiconductorfield effect transistors (MOSFETs).

FIG. 1 illustrates a simplified cross-sectional view of a lateral DMOStransistor with a dummy gate in accordance with an embodiment. Thelateral DMOS transistor 100 includes a substrate 102, preferably P-type,an N-type region 104 formed over the substrate 102, a P-type region 106and a first N+ region 116 formed over the N region 104. A P+ region 112and a second N+ region 114 are formed in the P-type region 106. Thelateral DMOS transistor 100 further comprises a gate dielectric layer122 with a first thickness and a dielectric insulating layer 124 with asecond thickness. An active gate 134 and a dummy gate 136 are formed onthe gate dielectric layer 122 and the dielectric insulating layer 124respectively.

The substrate 102 may be formed of silicon, silicon germanium, siliconcarbide or the like. The N-type region 104 is formed over the substrate102. In accordance with an embodiment, the N-type region 104 may be anepitaxial layer. The N-type region 104 may be doped with an n-typedopant such as phosphorous to a doping density of about 10¹⁵/cm³ to10¹⁸/cm³. It should be noted that other n-type dopants such as arsenic,nitrogen, antimony, combination thereof, or the like, couldalternatively be used. It should further be noted that the N-type region104 of the LDMOS 100 may be alternatively referred to as an extendeddrift region.

The P-type region 106 is formed by implanting p-type doping materialssuch as boron, gallium, aluminum, indium, combinations thereof, or thelike. In accordance with an embodiment, a p-type material such as boronmay be implanted to a doping density of about 10¹⁵/cm³ to 10¹⁸/cm³.Alternatively, the P-type region 106 can be formed by a diffusionprocess. The P-type region 106 of the LDMOS 100 may be alternativelyreferred to as a channel region.

A first N+ region 116 is formed in the N-type region 104. In accordancewith an embodiment, the first N+ region 116 functions as a drain of theLDMOS 100. The drain region may be formed by implanting an n-type dopantsuch as phosphorous at a concentration of between about 10¹⁹/cm³ andabout 10²⁰/cm³. Furthermore, a drain contact 138 is formed over thefirst N+ region 116.

A second N+ region 114 is formed in the P-type region 106. In accordancewith an embodiment, the second N+ region 114 may be a source of theLDMOS 100. The source region may be formed by implanting an n-typedopant such as phosphorous at a concentration of between about 10¹⁹/cm³and about 10²⁰/cm³. As shown in FIG. 1, the source region is formedadjacent to the active gate 134 on the opposite side from the drain (thefirst N+ region 116).

A P+ region 112 is formed adjacent to the second N+ region 114 in theP-type region 106. The P+ region may be formed by implanting a p-typedopant such as boron at a concentration of between about 10¹⁹/cm³ andabout 10²⁰/cm³. The P+ region 112 may contact the p-type body. In orderto eliminate the body effect, the P+ region 112 may be coupled to thesource 114 directly through the source contact 132.

A gate dielectric layer 122 is formed over the N-type region 104. Thegate dielectric layer 122 may be formed of silicon oxide, siliconoxynitride, hafnium oxide, zirconium oxide or the like. As shown in FIG.1, the gate dielectric layer 122 is partially on top of the P-typeregion 106. In accordance with an embodiment, the gate dielectric layer122 is of a thickness of between about 100 Å and about 200 Å. An activegate 134 is formed on the gate dielectric layer 122. The active gate 134may be formed of polysilicon, polysilicon germanium, nickel silicide orother metal, metal alloy materials.

The dielectric insulating layer 124 is on top of the N-type region 104.More particularly, the dielectric insulating layer 124 is locatedbetween the active gate 134 and the drain 116. In accordance with anembodiment, the dielectric insulating layer 124 is of a thickness ofbetween about 500 Å and about 2000 Å. A dummy gate 136 is formed on topof the dielectric insulating layer 124. It should be noted that thedummy gate 136 may be formed in the same process step as the active gate134. More particularly, the dummy gate 136 and the active gate 134 maybe formed as a large single gate first. In order to reduce the gate todrain charge, a middle portion of such a large single gate may beremoved by using suitable etching techniques such as dry etching. As aresult, the remaining portions of the large single gate become theactive gate 134 and the dummy gate 136 respectively.

The dummy gate 136 functions as a field plate, which helps to maintainthe breakdown voltage of the LDMOS 100. In addition, by separating thedummy gate 136 from the active gate 134, the overlap between the gateregion and drain region is reduced accordingly. Such a small overlapbetween gate region and drain region helps to reduce the gate-to-draincharge of the LDMOS 100. Furthermore, the dummy gate 136 may be biasedby a voltage source. For example, when the LDMOS 100 is a 20Vtransistor, the bias voltage coupled to the dummy gate 136 may be up to20V. Such a bias voltage helps to reduce the on resistance of the LDMOS100.

The first isolation region 142 and the second isolation region 144 areused to isolate active regions so as to prevent leakage current fromflowing between adjacent active regions. The isolation region (e.g.,142) can be formed by various ways (e.g., thermally grown, deposited)and materials (e.g., silicon oxide, silicon nitride). In thisembodiment, the first isolation region 142 and the second isolationregion 144 may be fabricated by a shallow trench isolation (STI)technique.

FIG. 2 illustrates a simplified top view of a lateral DMOS transistorwith a dummy gate in accordance with an embodiment. In FIG. 2, theN-type region 104, the first N+ region 116, the second N+ region 114,the P+ region 112, the active gate 134 and the dummy gate 136 are shown.In addition, dimensions of the LDMOS transistor 100 are shown. Inparticular, the distance between the active gate 134 and the dummy gate136 is illustrated. In accordance with an embodiment, the distance D1 isa range from 0.1 um to 1 um.

One skilled in the art will recognize that FIG. 1 illustrates an idealprofile as doped. The distance D1 may vary after subsequent diffusionprocesses. The distance D1 shown in FIG. 2 is used to illustrate theinventive aspects of the various embodiments. The present invention isnot limited to any particular distance between the active gate 134 andthe dummy gate 136.

FIG. 3 illustrates two curves illustrating the performance differencebetween a traditional LDMOS transistor without a dummy gate and LDMOStransistor with a dummy gate. The horizontal axis of FIG. 3 representsthe breakdown voltage across an LDMOS transistor. The vertical axis ofFIG. 3 represents a performance index of the LDMOS transistor. Moreparticularly, the performance index is the produce of On-resistance(R_(ON)) and gate-to-drain charge (Q_(GD)). In accordance with anembodiment, the conduction losses of an LDMOS transistor may beproportional to the on resistance of the LDMOS transistor. On the otherhand, the switching losses of the LDMOS transistor may be proportionalto the gate-to-drain charge of the LDMOS transistor. In order to achievean efficient LDMOS transistor, a low R_(ON)×Q_(GD) is desirable forreducing conduction losses as well as switching losses.

A curve 302 and a curve 304 illustrate the R_(ON)×Q_(GD) differencebetween a traditional LDMOS transistor without a dummy gate and LDMOStransistor with a dummy gate. As shown in FIG. 3, at the same breakdownvoltage level, an LDMOS transistor with a dummy gate is of a lowR_(ON)×Q_(GD). For example, when the dummy gate LDMOS transistor has a31V breakdown voltage, the corresponding R_(ON)×Q_(GD) is about 10mOhm-nC. By contrast, with the same breakdown voltage level, the LDMOStransistor without a dummy gate has a higher R_(ON)×Q_(GD). Asillustrated by curve 302, the R_(ON)×Q_(GD) of the traditional LDMOStransistor is about 20 mOhm-nC.

FIG. 3 shows an LDMOS transistor having a dummy gate may achieve ahigher breakdown voltage without sacrificing its R_(ON)×Q_(GD). As such,an advantageous feature of this embodiment is that a dummy gate helps tomaintain the breakdown voltage of an LDMOS transistor. In addition, thedummy gate can reduce the gate charge of the LDMOS transistor so thatthe switching losses of the LDMOS transistor may be reduced.

Although embodiments of the present disclosure and its advantages havebeen described in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the disclosure as defined by the appendedclaims.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the present disclosure, processes, machines,manufacture, compositions of matter, means, methods, or steps, presentlyexisting or later to be developed, that perform substantially the samefunction or achieve substantially the same result as the correspondingembodiments described herein may be utilized according to the presentdisclosure. Accordingly, the appended claims are intended to includewithin their scope such processes, machines, manufacture, compositionsof matter, means, methods, or steps.

What is claimed is:
 1. A semiconductor device comprising: an extendeddrift region over a substrate; a drain region in the extended driftregion; a channel region in the extended drift region; a source regionin the channel region; a first dielectric layer with a first thicknessformed over the extended drift region; a second dielectric layer with asecond thickness formed over the extended drift region and the channelregion, wherein the first thickness is greater than the secondthickness, and wherein the first dielectric layer and the seconddielectric layer form two steps, and wherein a first step is over a topsurface of the extended drift region and at least a portion of a secondstep is below the top surface of the extended drift region; a first gateformed over the first dielectric layer; and a second gate formed abovethe second dielectric layer, wherein the first step is between the firstgate and the second gate and the first step is free of a gate materialthereon.
 2. The semiconductor device of claim 1, wherein: the seconddielectric layer extends from an edge of the source region to an edge ofthe first dielectric layer; and the first dielectric layer extends froman edge of the second dielectric layer to an edge of the drain region.3. The semiconductor device of claim 1, wherein: at least a portion ofthe first dielectric layer is below the top surface of the extendeddrift region.
 4. The semiconductor device of claim 1, wherein: thechannel region is p-type; the drain region is n-type; and the sourceregion is n-type.
 5. The semiconductor device of claim 4, furthercomprising: a p-type region between and in contact with an isolationregion and the source region.
 6. The semiconductor device of claim 5,wherein: the p-type region and the source region share a same contact.7. The semiconductor device of claim 1, wherein: the first gate is adummy gate; and the second gate is an active gate.
 8. The semiconductordevice of claim 7, wherein: the first gate is configured to be biased bya voltage source.
 9. The semiconductor device of claim 8, wherein: abias voltage supplied by the voltage source is substantially equal to arated voltage of a transistor formed by the drain region, the sourceregion, the first gate and the second gate.
 10. A device comprising: afirst drain/source region and a second drain/source region over asubstrate; a first gate over a first dielectric layer; and a second gateover a second dielectric layer, wherein: a thickness of the seconddielectric layer is greater than a thickness of the first dielectriclayer; the first dielectric layer and the second dielectric layer form astep; the first dielectric layer extends from an edge of the firstdrain/source region to the step; the second dielectric layer extendsfrom the step to an edge of the second drain/source region; and the stepis free of a gate material thereon.
 11. The device of claim 10, wherein:the first drain/source region, the second drain/source region, the firstgate and the second gate form a lateral double diffused metal oxidesemiconductor (LDMOS) device, and wherein: the first drain/source regionis a source; the second drain/source region is a drain; the first gateis an active gate; and the second gate is a dummy gate.
 12. The deviceof claim 10, further comprising: an epitaxial layer over the substrate;and a channel region in the epitaxial layer, wherein: the firstdrain/source region is in the channel region; and the seconddrain/source region is in the epitaxial layer.
 13. The device of claim10, further comprising: a first contact formed over the firstdrain/source region; and a second contact formed over the seconddrain/source region.
 14. The device of claim 13, further comprising: abody contact region between and in contact with an isolation region andthe first drain/source region, wherein the body contact region iscoupled to the first drain/source region through the first contact. 15.The device of claim 10, wherein: an edge of the first dielectric layeris vertically aligned with an edge of the first drain/source region; andan edge of the second dielectric layer is vertically aligned with anedge of the second drain/source region.
 16. A transistor comprising: anextended drift region over a substrate; a drain region in the extendeddrift region; a channel region in the extended drift region; a sourceregion in the channel region; a first dielectric layer with a firstthickness formed over the extended drift region; a second dielectriclayer with a second thickness formed over the extended drift region andthe channel region, wherein: the first thickness is greater than thesecond thickness; at least one portion of the first dielectric layer isbelow a top surface of the extended drift region; and a boundary betweenthe first dielectric layer and the second dielectric layer is free of agate material thereon; a first gate on the first dielectric layer; and asecond gate on the second dielectric layer.
 17. The transistor of claim16, wherein: a width of the second dielectric layer is greater than awidth of the second gate; and the second gate occupies a middle portionof the second dielectric layer.
 18. The transistor of claim 16, wherein:a bottom surface of the first dielectric layer is lower than the topsurface of the extended drift region with reference to a top surface ofthe substrate.
 19. The transistor of claim 18, further comprising: abody contact region between and in contact with an isolation region andthe source region.
 20. The transistor of claim 16, wherein: the firstgate is configured to be biased by a bias voltage.